The invention relates generally to data transmission techniques and, more particularly, to a method and system for combining multiple individual asynchronous data streams for simultaneous transmission via a single conductor or wireless transmission medium.
There are at least two known techniques for achieving the transmission of multiple data streams in a single medium. The first is xe2x80x9ctime domain multiplexingxe2x80x9d, or xe2x80x9cTDMxe2x80x9d, in which each of the multiple individual data streams to be combined is sampled and assigned a specific timed space in a new data stream. This new data stream must also include some form of synchronization information that serves to identify the beginning of the data packages.
TDM requires that the new data stream be faster than any of the individual data streams being multiplexed. Typically, the data sampling rate is eight to ten times that of the highest speed data stream to be sampled to reduce the jitter or time errors incurred during the sampling process. Synchronicity must also be addressed. Specifically, the addition of overhead for the synchronization information runs the bandwidth of the new data stream to a much higher rate than any of the individual data streams. Moreover, prudent following of the Nyquist sampling requirements would suggest that the actual individual sampling rate for any data stream must be twice that of the sampled data stream, putting the actual data rate to as much as 24 times the highest individual rate (ten channels, two bits for sync, two times over-sampling).
TDM is illustrated in FIG. 1. FIG. 1 illustrates a sequential series of sampled data bits numbered 1 through 10. Two additional bits of data space are employed as a synchronization (xe2x80x9csyncxe2x80x9d) signal. Synchronization is usually accomplished using a special bit format that is clearly not data. This violation of the data convention obviously isolates the synchronization event. As shown in FIG. 1, ten samples became twelve bits. Assuming a data rate of 10 Mbits/second, the new data stream is at 120 Mbits/second (12*10 Mbits/second). Two bits equal a hertz, making the signal a 60 MHZ rates. Assuming a ten-to-one bandwidth requirement to send square waves results in a 600 MHZ bandwidth requirement to transfer the ten channels.
The second technique is known as frequency domain multiplexing (xe2x80x9cFDMxe2x80x9d), in which each individual data stream to be combined is used to modulate a different individual carrier using either amplitude or frequency modulation. The modulated carriers are then transmitted in a manner similar to that used by individual radio stations in making transmissions. As illustrated in FIG. 2, this technique requires that the total bandwidth be sufficiently wide to accommodate the sum of the carriers and their respective data-bearing sidebands.
Clearly, FDM also poses a bandwidth problem. In particular, the two primary issues that must be addressed are the selection of an appropriate frequency for the modulating carrier to provide clean sidebands of modulated signal, and the actual width of the sidebands. Referring again to FIG. 2, using the same data assumptions as the TDM example described with reference to FIG. 1, ten channels of 10 Mbits each would require using 20 MHZ of bandwidth for each data-carrying sideband. Adding 5 MHZ for a guard band between each of the channels and approximately 35 MHZ of start frequency results in a bandwidth of approximately 270 MHZ for the 10 channels.
It is apparent that the primary disadvantage of both of the techniques described above is the large bandwidth needed to transmit the signals. With TDM, the bandwidth requirement is due to the data sampling process itself and the overhead required for synchronization in the demultiplexing process. With FDM, the bandwidth requirement is due to the spectrum spread needed for the individual carriers and their respective sidebands.
Additional disadvantages associated with TDM include the fact that some form for sampling synchronization must exist with the data. If the data itself is not synchronous with the sampling clock, then some additional circuitry must be employed to convert the asynchronous data to synchronous. Typically, this would be through the use of FIFOs or active memory and a reclocking scheme. Clearly, implementation of TDM is costly and complex.
FDM suffers from the same disadvantages associated with TDM, as described above.
The deficiencies of TDM and FDM can be summarized as follows:
1. a limited amount of digital data can be transferred in a given bandwidth;
2. wide bandwidth is required to achieve high digital data transfer rates;
3. the cost in materials, space, heat, and power required to transfer the data is great;
4. the distance that the data can be transferred is limited;
5. the quality and age of the medium infrastructure can negatively affect the data; and
6. external noise can negatively affect the data.
Therefore, what is needed is a technique for combining multiple asynchronous data signals in a minimum of bandwidth.
In a preferred embodiment, the invention is a method and system for combining multiple individual asynchronous data streams for simultaneous transmission in the analog domain via a single conductor or wireless transmission medium.
In one embodiment, a carrier signal is modulated and demodulated on a half-cycle basis. Each half-cycle is amplitude modulated (i.e., multiplied) by a fixed value representative of the data to be encoded that is applied to the half-cycle at zero-crossing and is held steady for the duration of the half-cycle. In this manner, each half-cycle of a carrier signal is modulated to contain data. For purposes of redundancy or security, two or more half-cycles may be used to contain the data, but in each case, the modulation still occurs on a half-cycle basis.
As previously indicated, the actual change in the modulation must occur at the zero-crossing point of the carrier, such that the carrier has, for the half-cycle, its level set by a constant rather than by a signal whose amplitude changes over the time of the half-cycle.
A technical advantage achieved with the invention is that multiple digital data signals may be combined and transmitted in the analog domain.
Another technical advantage achieved with the invention is that the bandwidth of the data encoded and transferred using the method and system of the invention is considerably smaller than that of data encoded using TDM or FDM.
Yet another technical advantage achieved with the invention is that it is implemented using considerably simpler and less expensive circuitry for the data rates handled.